Most automotive power electronics systems must comply with standards for transient immunity. One such standard is ISO 7637-1. ISO 7637-1 is an over voltage protection standard.
FIG. 1 shows a simulation of a transient condition which occurs when the car battery is disconnected while the car alternator is generating charging current and the loads remain connected to the alternator. The transient condition shown by FIG. 1 is conventionally referred to as a load dump event. Load dump may occur due to a fortuitous event such as battery cable corrosion, poor cable connection or the disconnection of the battery while the engine is running.
The amplitude of a load dump transient pulse depends on the alternator speed and on the level of the alternator field excitation at the moment of the battery disconnection, while the duration of the pulse depends essentially on the time constant of the field excitation circuit and on the amplitude of the pulse.
The following are the relevant parameters for understanding the load dump event illustrated by FIG. 1.
Vs=+26.5 V to +86.5V
Ri=0.5 Ω to 4 Ω
td=40 ms to 400 ms
tr=5 ms to 10 ms
It should be noted that the internal resistance Ri of an alternator, in case of load dump, is mainly a function of the rotational speed of the alternator and the excitation current as seen in the following relationship:
      R    i    =            10      ×              V        nom            ×              N        act                    0.8      ×              I        rated            ×      12000      ⁢                          ⁢              min                  -          1                    where
Vnom is the specified voltage of the alternator;
Irated is the specific current at an alternator speed of 6000 min−1 (as given in ISO 8854); and
Nact is the actual alternator speed, in reciprocal minutes.
Also, it should be noted that the pulse is determined by the peak voltage Vs, the internal resistance Ri, and the pulse duration td. In all cases, small values of Vs are correlated with small values of Ri and td, and high values of Vs with high values of Ri and td. The latter parameters define the dynamic behavior of an alternator during load dump.
Ideally, electronic devices in an automobile that can be damaged by the high voltage generated during a load dump event are protected. FIG. 2 shows a conventional voltage suppression technique for preventing damage to an electronic control unit (ECU) 10 during a load dump event. The ECU may be a control circuit for controlling the operation of an electric motor. As shown by FIG. 2, Zener diode 12 is used for voltage suppression of a transient voltage generated during a load dump event. The load dump event is schematically illustrated by load dump generator 14. Zener diode 12 imposes a constant voltage on the terminals of ECU 10, and sinks the required amount of current to drop the exceeding voltage on the source impedance. In the circuit shown by FIG. 2, the breakdown voltage of Zener diode 12 is chosen in view of the voltage that the circuit can withstand. The higher the breakdown voltage of ECU 10, the less energy Zener diode 12 has to dissipate. Hence, Zener diode 10 may be smaller.
The use of Zener diodes in voltage suppression presents the designer with certain challenges. For example, the breakdown voltage of Zener diodes change over temperature, which requires the designer to be mindful of temperature changes and may even restrict the use of ECU 10 to environments in which temperature changes are modest or controlled. Also, Zener diodes add space and cost to any system and require large tracks on the circuit board to carry high current. Thus, it is desirable to eliminate the Zener diode.
Another solution to the load dump problem is to increase the rated voltage of all the to-be-protected electronic components. Unfortunately, increasing the breakdown voltage of electronic components decreases their performance, thus requiring larger and more expensive components to be used.